1. Field of the Invention
This invention is related to the production of thermoplastic foam. More particularly, it is related to the production of thermoplastic film or sheeting having an integral skin layer on the surface of film or sheeting.
2. Description of the Prior Art
It is known to extrude certain thermoplastic resins directly into foam-form film or sheeting by plastifying the resin, blending it with a suitable nucleating agent and blowing agent, and extruding this blend through an appropriate size die under such conditions as to form the extrudates into film or sheet form while foaming it. In one particular modification of this generally known process, the blend is extruded through a circular (annular) die gap to produce a tubular foam-form film or sheeting extrudate. The tubular extrudate is then passed over a forming mandrel having a cooling fluid passing therethrough. In this process, it is common for the forming mandrel to have such a diameter as to necessitate the tubular film being stretched to fit over the mandrel.
After the tubular foam-form extrudate passes over the forming mandrel and is cooled to a sufficient extent so that the foam form character thereof is substantially stable, the tube is conventionally slit in the longitudinal direction, spread flat and wound on a take-up roller means. It is known that the stretching of the tubular extrudate over the forming mandrel and/or the winding of the flattened foam-form material on the take up means causes the thus formed film or sheeting to have been oriented in the longitudinal and transverse directions.
Attempts have been made in the past to produce such thermoplastic foam-form film or sheeting having a skin layer on the surface thereof. The skin layer is unfoamed, while the underlying thermoplastic film or sheeting is foamed. Such a skin layer improves mechanical properties of the thermoplastic film or sheeting. The skin layer also provides a better printing surface than the foamed film or sheeting underneath the skin layer. Several different methods have been known for producing such an outer skin layer. For example, Lux, U.S. Pat. No. 3,299,192, discloses the formation of such a skin layer on the thermoplastic foamed pipe by quench chilling the internal and external walls of the tube a short time after it emerges from an extrusion die. Johnson et al, U.S. Pat. No. 4,352,893, disclose the production of superior styrenic polymer foam sheets exhibiting excellent resistance to punctures and improved tensile properties by cooling the extruded foam sheets in a water bath at a temperature from about 40.degree. C. to about 85.degree. C. It has also been proposed to form the outer skin layer by air cooling the surface of the extruded foamed sheet.
One of the potential problems in the production of the skin layers by the aforementioned methods is that the thermoplastic article, i.e., foam-form film or sheeting is later subjected to thermoforming conditions to produce a desired article, as discussed above. During such thermoforming conditions, the article is subjected to elevated temperatures which may promote the foaming of the blowing agent contained in the film surface.
Other methods have also been proposed to obtain a foam-form film or sheeting containing an outer skin layer. For example, it has been proposed to extrusion-coat a non-foaming resin onto a simultaneously-extruded thermoplastic foam-form film or sheeting. In such an operation, the non-foaming resin is coextruded simultaneously with the foamed film or sheeting in the same extruder or through two different extruders. However, one of the disadvantages of the resulting article is that the film surface is non-oriented and, therefore, it exhibits inferior mechanical properties as compared to oriented articles.
Another previously-proposed method for providing a foam-form film or sheeting having a surface skin layer was heat-lamination of the skin layer onto the foam-form film or sheeting, each previously produced independently of each other. In such a process, two separate process operations are necessary for the production of the article having an integral skin surface, thereby contributing to the high overall cost of the final article.
Accordingly, it is a primary object of the present invention to provide an improved process and apparatus for producing thermoplastic foam-form film or sheeting having an integral skin.
Additional objects of this invention will be apparent to those skilled to the art from the following description thereof.